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Ingredient guide

Thiamine side effects: what studies and labels report

Written by the Full Measure editorial team, which sells supplements. Approved for publication by the Full Measure owners. Updated .

Drafted with AI assistance from the sources listed below. How we write these

Quick answer

Thiamine taken by mouth has not been shown to cause harm. NIH reports no adverse effects from intakes of 50 mg a day or more, so no upper limit is set. Rare but serious allergic reactions are linked to thiamin injections, and two medicines, furosemide and fluorouracil, can lower thiamin levels.1,2,3

Key facts

  • In one early report, nausea and indigestion appeared in 2 people only at 7,000 mg a day or more.4
  • In 989 people given thiamin into a vein, 1.1% had a reaction, almost all brief irritation at the site.5
  • Absorption drops off above about 5 mg a day, one likely reason high intakes appear harmless.2
  • In older adults the usual concern is too little thiamin, not too much.2

Thiamine by mouth

No upper limit has been set for thiamin, because no harm has been reported from high intakes. Absorption falls above about 5 mg a day, and what the body does not need leaves in urine.2,6

Earlier reports summarized in a 2012 study describe oral doses of 3,000 mg a day or more taken for long periods without harm, and nausea and indigestion in 2 people only when doses reached 7,000 to 7,500 mg a day.4

The Food and Nutrition Board, which sets US intake values, thinks the lack of harm comes from the gut's limited uptake: above about 5 mg, absorption falls off sharply. It still cautions that very high intakes could have effects that have not been reported.2

The 2012 pharmacokinetic study gave single doses of up to 1,500 mg and reported no side effects, but it was not designed to detect them, so it shows tolerance in a small group rather than proof of safety.4

Thiamine injections

Thiamin given by injection can rarely cause a serious allergic reaction (anaphylaxis), most often after repeated injections, and deaths have been reported. Warmth, itching, hives, weakness, sweating and nausea have also been reported.3

In a study of 989 people given 100 mg of thiamin into a vein, 1.1% had a reaction: 11 had brief irritation where it went in and 1 had itching all over.5

Injections are given in clinics and hospitals, mostly to people with a confirmed or suspected deficiency who cannot rely on tablets. The label tells staff to watch for reactions and to test first in anyone with a history of drug allergy or an earlier reaction to thiamin.3

Medicines that lower thiamin

NIH says thiamin itself is not known to interact with medicines. The concern runs the other way: some medicines lower thiamin levels.2

Your doctor may want to check on you

Furosemide, a loop diuretic, has been linked in research to lower thiamin levels, possibly low enough to be deficient, because more thiamin is lost in urine. NIH says people who take it regularly should discuss their thiamin status with their health care providers.1,2,7

Reported in studies

Your doctor may want to check on you

Fluorouracil, a chemotherapy medicine, has been linked in published cases to severe thiamin deficiency, including beriberi and Wernicke's encephalopathy, possibly because it speeds up thiamin breakdown and blocks its active form. NIH notes that thiamin supplements might reverse some of these effects; that is a decision for the cancer care team.1,2

Reported in studies

Older adults

Up to 20% to 30% of older adults have lab results that suggest some degree of low thiamin status.2

Low intake, several long-term illnesses, many medicines and weaker absorption with age can all play a part, and people living in care homes appear most affected. For an older adult, the useful question for a doctor is usually whether thiamin is low, not whether a supplement is too much.2

Questions people ask

Is it safe to take thiamine every day?

NIH reports no adverse effects even from intakes of 50 mg a day or more, and no upper limit is set. The body needs some every day because it stores little. Ask your doctor first if you take furosemide or fluorouracil, or if you are pregnant and vomiting often.2

Does thiamine cause weight gain or change urine color?

None of the official sources or studies we read lists weight gain as a side effect of thiamin. Thiamin itself is a white or colorless powder; the bright yellow urine some people notice after a B vitamin comes from riboflavin.3,8

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Sources

  1. Office of Dietary Supplements, National Institutes of Health. Thiamin: Fact Sheet for Consumers. Updated March 22, 2021. Opened 2026-10-07.
  2. National Institutes of Health, Office of Dietary Supplements. Thiamin: Fact Sheet for Health Professionals. Updated February 9, 2023. Opened 2026-10-07.
  3. Thiamine hydrochloride injection: prescribing information (Lifestar Pharma). DailyMed, U.S. National Library of Medicine. Published October 1, 2026. Opened 2026-10-07.
  4. Smithline HA, Donnino M, Greenblatt DJ. Pharmacokinetics of high-dose oral thiamine hydrochloride in healthy subjects. BMC Clin Pharmacol. 2012;12:4. Opened 2026-10-07.
  5. Wrenn KD, Murphy F, Slovis CM. A toxicity study of parenteral thiamine hydrochloride. Ann Emerg Med. 1989;18(8):867-870. Opened 2026-10-07.
  6. EFSA Panel on Dietetic Products, Nutrition and Allergies (NDA). Dietary reference values for thiamin. EFSA J. 2016;14(12):e04653. Opened 2026-10-07.
  7. Rieck J, Halkin H, Almog S, et al. Urinary loss of thiamine is increased by low doses of furosemide in healthy volunteers. J Lab Clin Med. 1999;134(3):238-243. Opened 2026-10-07.
  8. National Institutes of Health, Office of Dietary Supplements. Riboflavin: Fact Sheet for Health Professionals. Updated May 11, 2022. Opened 2026-10-07.

This page is information, not medical advice. Talk to your doctor or pharmacist about your own situation.